1. Field of the Invention
The present invention relates to a low-pressure rare gas discharge lamp used for an OA apparatus such as a facsimile (FAX) machine, a copying machine, etc., and particularly to a method of lighting a hot-cathode type low-pressure rare gas discharge lamp including using at least one of electrodes provided at each end of a glass valve as a hot cathode operated in a stable discharge state, and emitting light from a fluorescent material with ultraviolet rays generated by discharge of the low-pressure rare gas sealed in the glass valve, or with visible light, directly
2. Description of the Related Art
A lamp which employs light emission of a discharge in a rare gas has recently been used as a light source for an OA apparatus. However, practical use of such a lamp, requires preventing distortion of the luminance distribution.
For example, moving stripes occur which are peculiar to a low-pressure rare gas discharge, as disclosed in Extended Abstracts No. 57 of the National Convention in the 75th Anniversary of the Founding of Illumination Society (24th), p. 84, 1991, the Committee/Executive Committee of the National Convention of Illumination Society.
This is described in detail in Technical Report of Mitsubishi Denki, 65 (4), pp. 82-86, 1991. Since bright and dark portions of a discharge positive column irregularly move to form stripes in the axial direction of the lamp, when the lamp is used as a reading light source for an OA apparatus, the moving stripes cause an instantaneous distortion in the luminance distribution, and adversely affects reading in some cases.
A direct current lighting method is thus proposed to solve the above problem. In this method, since groups of stripes having the same size regularly occur in the lamp and move at a high speed in one direction, a reading CCD is uniformly affected by the moving stripes during reading of an original in spite of the occurrence of the moving stripes, and thus the amount of light received is made uniform, thereby preventing the adverse effects of the moving stripes.
On the other hand, Japanese Patent Laid-Open No. 1-157053 discloses means for heating a hot cathode to a temperature within the range of 800.degree. C. to 1200.degree. C. during lighting so as to improve the luminance distribution.
The high-speed operation of recent OA apparatus decreases the one-line reading time of FAX, and brings about a demand for higher stability of the light output of a lamp due to an increase in the required gradation number, i.e., gray scale.
However, although not all lamps produce variations in light quantity other than the moving stripes, i.e., variations in light quantity (referred to as flicker hereinafter) for a longer period of time than the moving stripes, which has not been reported as yet, some lamps irregularly produce flicker.
With a long reading time and many irregularities, a change in the total amount of light received for one line due to flicker is not a problem. However, in an apparatus having a high-operation speed and a short reading time, such as recent OA apparatus, the adverse effects on reading due to flicker other than the moving stripes cause a critical problem.
Another conceivable factor which further increases the problems caused by flicker is an increase in the number of reading gradations. If a simple decision is made as to whether each portion of one reading line is black or white, a variation in light quantity is not a critical problem. However, when the number of gray scale gradations is as large as 256, a light source must have a stability of light output corresponding to a variation of 1/256 or less per reading time in order to read a density difference of 1/256 of an original.
In addition, flicker does not always occur during lighting of the lamp, as described above, and cannot be controlled from outside of the apparatus.